1. Field of the Invention
This invention relates generally to RF systems which perform radar, electronic warfare, and communication/navigation/identification functions and, more particularly, to integrated avionic type RF systems which perform such functions using common hardware.
2. Description of the Prior Art
Current RF avionics systems for performing radar, electronic warfare(EW) and communications/navigation/ identification(CNI) functions are typically comprised of multiple systems segregated by their individual functions. In such systems, any one system performs only a few functions, such as landing an aircraft or performing electronic countermeasures. More recently, however, the state of the art has moved toward system integration where all tactical RF avionics are included in three distinct functional systems, namely radar, EW, and CNI. Integration has concentrated within the distinct boundaries of these three specific functions.
Each of the aforementioned systems has its own means of implementing receiver and exciter resources and employ respective superhetrodyne frequency conversions to a common intermediate frequency(IF), thereby enabling common receivers and exciters to be used over a wide frequency range. Design of this hardware requires optimization of IF frequency selections to maximize performance with conventional components. However, the differing requirements of each of the three systems has resulted in different choices for their IF frequency conversion implementations.
Within an integrated RF avionic system, the key receive resources are frequency converters and IF receivers. The ability to share these resources permits dynamic reconfiguration to increase the available quantity and provide fault tolerance. Sharing common receive resources permits functions to be supported that require large number of simultaneous receive channels such as monopulse tracking, adaptive antenna pattern control, and passive direction finding. In stand-alone systems, these functions may not be available due to the size, weight and cost. Using common resources also reduces the number of individual unique modules, thus reducing development cost and enhancing supportability. To enable the sharing of frequency converters in IF receivers, a common IF interface along with appropriate RF and IF switches are essential. However, a common first IF frequency, although beneficial from the standpoint of an integrated RF avionics system, presents two major obstacles, namely: (1) ideal frequency selections are diametrically opposing; and (2) a common IF aggravates the need for high signal isolation. Accordingly, all integrated RF sensor systems present key obstacles that must be addressed. The various functions have differing requirements which include: frequency coverage, instantaneous bandwidth and dynamic range.
Prior attempts in addressing this challenge have concentrated on two methods of integration, namely (1) converting all signals to a common IF frequency, or (2) using separate frequency conversion resources with separate IF signal channels.
Converting all signals to a common IF frequency provides the simplest solution in terms of interconnection between modules because then a single IF port can be used to interface the various modules. These interfaces are implemented by switch networks which reconfigure the system for various functions. However, a common IF frequency selection involves some compromise in performance, particularly where radar and EW functions are integrated. Unfortunately, radar and EW introduce diametrically opposing requirements that inhibit selection of a suitable common IF. This is because the driving factor for IF selection for EW functions is providing wide instantaneous bandwidth. This requires a relatively high IF frequency. A sufficiently high IF band is selected for EW functions in order to avoid generation of signal harmonics within the band extent which may be misinterpreted as real emitters by the receiver circuitry. On the other hand, the driving factor for radar IF selection is minimizing spurious generation by mixers. This is accomplished by maintaining a sufficiently high ratio between the RF and IF frequencies. Accordingly, in order to avoid low order spurious responses, a relatively low IF frequency is required.
Because common first IF frequency selection requirements for EW and radar are diametrically opposing, their integration into common hardware is difficult since, as noted above, radar functions demand a relatively low IF, while the EW functions demand a relatively high IF. The use of separate frequency conversion resources to optimize performance of individual functions circumvents the loss of performance due to a compromised IF. Converting all signals to a common IF, however, introduces a new challenge of providing sufficient isolation so as to eliminate cross-talk between functions.
Furthermore, as system resources are placed into a common rack and shared between functions, there are several paths for cross-talk when all functions use the same IF. These typically include: leakage between RF connectors, leakage onto power and control signals, and finite isolation of switch networks employed in reconfiguring system resources. Radar reception requires very high instantaneous dynamic range to detect small targets in the presence of strong ground clutter. EW signals can be quite strong in amplitude, since they only incur a one-way propagation loss. The result is the need to achieve a very high signal isolation on the order of 80-100 dB when considering typical tactical requirements and thus presents an additional challenge to integrated sensor systems design.
Notwithstanding the inherent problems involved, emerging RF avionic system designs are pressing forward with integrated systems which will perform radar, EW and CNI functions which can share hardware resources. Such integrated designs enable receiver and exciter resources to be dynamically assigned to avionics functions based on flight and tactical mission needs. Advances in wideband RF technology and electronically scanned array antennas encourage full integration of their RF sensor resources. The benefit of an integrated RF system is that a limited number of resources can be made available to perform any function desired.